Steel for storage equipment and transportation equipment for ethanol (as amended)

ABSTRACT

Steel for storage equipment and transportation equipment for ethanol, the steel having a chemical composition containing, by mass %. C: 0.02% to 0.3%, Si: 0.01% to 1.0%, Mn: 0.1% to 2.0%, P: 0.003% to 0.03%, S: 0.01% or less, Al: 0.005% to 0.100%, N: 0.0010% to 0.010%, at least one selected from W: 0.010% to 0.5% and Mo: 0.010% to 0.5%, at least one selected from Sb: 0.01% to 0.5% and Sn: 0.01% to 0.3%, and the balance being Fe and inevitable impurities, in which the ratio of the Al content to the N content satisfies the relationship 2.0≤Al/N≤70.0.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2016/002938, filed Jun. 20, 2016, which claims priority to Japanese Patent Application No. 2015-124331, filed Jun. 22, 2015, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to structural steel which can preferably be used for the members of storage equipment and transportation equipment for ethanol. That is, the steel according to aspects of the present invention can preferably be used as steel for the members of storage equipment for ethanol and the members of transportation equipment for ethanol. In addition, the steel according to aspects of the present invention relates to structural steel excellent in terms of ethanol corrosion resistance which can be used in a corrosive environment in ethanol containing carboxylic acid, chloride ions, and water, in particular, in bioethanol.

BACKGROUND OF THE INVENTION

Bioethanol is manufactured mainly by degrading and refining the sugar of, for example, corn or wheat. Nowadays, bioethanol is widely used in the world as an alternative fuel to petroleum (gasoline) or as a fuel to be mixed with gasoline, and the amount of bioethanol used tends to increase year by year. Therefore, in a process of, for example, storing and transporting bioethanol or in a process of mixing bioethanol with gasoline, there is an increase in the amount of bioethanol handled. However, since bioethanol is highly corrosive, that is, since pitting corrosion, in particular, stress corrosion cracking (SCC) progresses, it is difficult to handle bioethanol.

The fact that a very small amount of acetic acid and chloride ions are mixed as impurities in bioethanol in a process for manufacturing bioethanol and the fact that bioethanol absorbs water and takes in dissolved oxygen in storage contribute to an increase in the corrosiveness of bioethanol. In particular, once SCC due to bioethanol occurs, there is a risk of causing a serious accident of bioethanol leakage. Therefore, the SCC due to bioethanol is a corrosive phenomenon which is regarded as the most serious problem, and it is considered to be important to prevent such SCC from occurring in service.

As described above, there is a disadvantage in that it is possible to safely handle bioethanol only by using equipment provided with protection against ethanol such as a tank constructed by using an organic coating material, stainless steel, or stainless-clad steel which is excellent in terms of ethanol corrosion resistance. In addition, there is a problem in that it is not possible for the transportation of bio ethanol to use, for example, a conventional pipeline which is used for the transportation of petroleum. As described above, there is still a problem in that equipment for handling bioethanol requires great cost.

As an example of a method for solving the problems described above, Patent Literature 1 proposes, as a measure to handle bio-fuel, a method in which a zinc-nickel coating layer as formed on a 5 mass % to 25 mass % of Ni containing steel material for a tank or in which a chemical conversion coating film containing no hexavalent chromium is formed on the above-mentioned coating layer. Patent Literature 1 states that this method provides good corrosion resistance in gasoline containing ethanol.

In addition, Patent Literature 2 proposes a steel material for a pipe excellent in terms of corrosion resistance for handing the vapor of a fuel such as bioethanol, which is manufactured by coating the surface of a steel plate with a Zn—Co—Mo coating layer in which the ratio of the Co content to the Zn content is 0.2 at % to 4.0 at %.

Patent Literature 3 reports a steel material excellent in terms of alcohol corrosion resistance containing, by mass %, Cr: 0.01% to 1.0% and two or all selected from Cu: 0.05% to 1.0%, Sn: 0.01% to 0.2%, and Ni: 0.01% to 1.0%.

In addition, in Non Patent Literature 1, the inhibitor effect of ammonium hydroxide against the SCC (stress corrosion cracking) of a steel material in a simulated solution of bioethanol has been discussed. Non Patent Literature 1 states that there is a decrease in the degree of SCC as a result of inhibiting crack growth by adding ammonium hydroxide in the simulated solution.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2011-26669

PTL 2: Japanese Unexamined Patent Application Publication No. 2011-231358

PTL 3: Japanese Unexamined Patent Application Publication No. 2013-129904

Non Patent Literature

NPL 1: F. Gui, J. A. Beavers, and N. Sridhar: Evaluation of ammonia hydroxide for mitigating stress corrosion cracking of carbon steel in fuel grade ethanol, NACE Corrosion. Paper, No. 11138 (2011)

SUMMARY OF THE INVENTION

The term “SCC” originally means a cracking phenomenon caused by the coaction of a corrosive environment and static stress. However, since bioethanol SCC is often observed in equipment which is subjected to a fluctuating load environment, bioethanol SCC is fundamentally regarded as a corrosion fatigue phenomenon. Corrosion fatigue which occurs under dynamic stress is a severe fracturing phenomenon in which crack growth occurs more rapidly under lower stress than in the case of SCC which occurs under static stress. That is, the present inventors considered that it is necessary to increase corrosion fatigue resistance in an ethanol environment in order to prevent bioethanol SCC.

It is considered that the Zinc-Nickel coating disclosed in Patent Literature 1 is effective for increasing corrosion resistance. Although such a Zn—Ni coating requires an electroplating treatment, there is no problem in the case of, for example, a small fuel tank for an automobile. However, in the case of a thick steel material having a thickness of 3 mm or more which is used for a large structure such as a storage tank having a capacity of 1000 kL or more or a line pipe, since there is a vast increase in treatment costs, it is not possible to use an electroplating treatment. In addition, in the case where, for example, a coating defect occurs, since pitting corrosion tends to progress in the defected portion on the contrary, corrosion fatigue tends to occur. Therefore, the Zinc-Nickel coating is not sufficient from the viewpoint of pitting corrosion resistance and corrosion fatigue resistance.

The Zn—Co—Mo coating disclosed in Patent Literature 2 also requires an electroplating treatment, and therefore it is not possible to use the coating for a thick steel material for a large structure for the same reason as in the case of Patent Literature 1. Also, the Zn—Co—Mo coating is not sufficient from the viewpoint of pitting corrosion resistance and corrosion fatigue resistance for the same reason as in the case of Patent Literature 1.

Although the steel material in Patent Literature 3 effective from the viewpoint of pitting corrosion resistance, corrosion fatigue resistance has not been discussed. Therefore, it is difficult to say that the steel material in Patent Literature 3 has satisfactory ethanol corrosion resistance which is required for a practical structure.

Moreover, although Non Patent Literature 1 states that the addition of an inhibitor certainly decreases the degree of a corrosion phenomenon such as corrosion fatigue, it is difficult to say that such an effect is sufficient. This is because an inhibitor develops its effect by being adsorbed onto the surface of a steel material and the adsorption behavior strongly depends on, for example, the pH of the environment. Therefore, in the case where corrosion occurs locally, there may be a case of insufficient adsorption. In addition, since there is a risk of environmental pollution due to the leakage of an inhibitor, it is difficult to say that the addition of an inhibitor is a preferable countermeasure against corrosion.

As described above, a method for preventing corrosion through the coating is not suitable for a large structure, and the addition of an inhibitor has an insufficient effect due to a variation in the effect of decreasing the degree of corrosion on the surface of structural steel. Therefore, there is a strong demand for steel for storage equipment and transportation equipment for ethanol excellent in terms of corrosion resistance, in particular, corrosion fatigue resistance in a bioethanol environment which includes carboxylic acid, chloride ions, and water as impurities.

An object according to aspects of the present invention is, by solving the problems with the conventional techniques, to provide structural steel for the members of storage equipment and transportation equipment for ethanol such as a steel pipe excellent in terms of ethanol corrosion resistance which can be used even in a bioethanol environment. The term “excellent in terms of ethanol corrosion resistance” here means a case where steel is excellent in terms of corrosion fatigue resistance in an ethanol environment which contains carboxylic acid, chloride ions, and water as impurities.

Solution to Problem

The present inventors, in order to solve the problems described above, diligently conducted investigations for the purpose of the development of steel for storage equipment and transportation equipment for ethanol excellent in terms of corrosion fatigue resistance in a bioethanol environment and, as a result, found that adding Mo and W is effective for decreasing the degree of corrosion fatigue in a bioethanol environment and that adding Sb and/or Sn and further Al in addition to Mo and W is effective. In addition, the present inventors found that there is a significant increase in corrosion fatigue resistance by decreasing the N content. Here, such effects are also effectively realized even in the case of SCC in a static load environment under a milder stress condition. Aspects of the present invention have been completed on the basis of the knowledge described above and additional investigations, and the subject matter of aspects of the present invention is as follows.

[1] Steel for storage equipment and transportation equipment for ethanol, the steel having a chemical composition containing, by mass %,

-   C: 0.02% to 0.3%, -   Si: 0.01% to 1.0%, -   Mn: 0.1% to 2.0%, -   P: 0.003% to 0.03%, -   S: 0.01% or less, -   Al: 0.005% to 0.100%, -   N: 0.0010% to 0.010%, -   at least one selected from W: 0.010% to 0.5% and Mo: 0.010% to 0.5%, -   at least one selected from Sb: 0.01% to 0.5% and Sn: 0.01% to 0.3%,     and the balance being Fe and inevitable impurities, in which the     ratio of the Al content to the N content satisfies the relationship     2.0≤Al/N≤70.0. -   [2] The steel for storage equipment and transportation equipment for     ethanol according to item [1], in which the steel has the chemical     composition. further containing, by mass %, at least one selected     from -   Cu: 0.05% to 1.0%, -   Cr: 0.01% to 1.0%, and -   Ni: 0.01% to 1.0%. -   [3] The steel for storage equipment and transportation equipment for     ethanol according to item [1] or [2], in which. the steel has the     chemical composition further containing, by mass %, at least one     selected from -   Ca: 0.0001% to 0.02%, -   Mg: 0.0001% to 0.02%, and -   REM: 0.001% to 0.2%. -   [4] The steel for storage equipment and transportation equipment for     ethanol according to any one of items [1] to [3], in which the steel     has the chemical composition further containing, by mass %, at least     one selected from -   Ti: 0.005% to 0.1%, -   Zr: 0.005% to 0.1%, -   Nb: 0.005% to 0.1%, and -   V: 0.005% to 0.1%. -   [5] The steel for storage equipment and transportation equipment for     ethanol according to any one of items [1] to [4], in which the steel     further has a tensile strength of 825 MPa or less and a yield     strength of 705 MPa or less.

According to the present invention, it is possible to obtain steel for storage equipment and transportation equipment for ethanol excellent in terms of ethanol corrosion resistance which can be used even in a bioethanol environment which contains carboxylic acid, chloride ions, and water. In the case where the present invention is used as steel for a storage tank, a transportation tank, and a pipeline structure for bioethanol, it is possible to use them for a longer time than ever, it is possible to avoid an accident caused by the leakage of bioethanol due to a corrosion fatigue phenomenon, and it is possible to provide them at low cost. Therefore, there is a significant effect on the industry.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereafter, the present invention will be specifically described.

The reasons for the above-described limitation on the range of the chemical composition of steel in accordance with aspects of the present invention will be described. Here, the contents of the constituent chemical elements of the chemical composition of steel are expressed in units of “mass %” in all cases, and “mass %” is referred to as “%” hereinafter, unless otherwise noted.

C: 0.02% to 0.3%

C: is a chemical element which is necessary for achieving satisfactory strength of steel, and the C content is set to be at least 0.02% in order to achieve preferable yield strength (350 MPa or more) and tensile strength (400 MPa or more) in accordance with aspects of the present invention. It is preferable that the C content be 0.03% or more. On the other hand, in the case where the C content is more than 0.3%, since there is a deterioration in weldability, there are limitations added when welding is performed. Therefore, the upper limit of the C content is set to be 0.3%. It is preferable that the C content be 0.20% or less. In accordance with aspects of the present invention, it is more preferable that the C content be 0.10% or less in order to obtain good corrosion fatigue resistance.

S: 0.01% to 1.0%

Si is added for the purpose of deoxidation, and there is an insufficient deoxidizing effect in the case where the Si content is less than 0.01%. On the other hand, in the case where the Si content is more than 1.0%, there is a deterioration in toughness and weldability. Therefore, the Si content is set to be 0.01% to 1.0%. Here, it is preferable that the lower limit of the Si content be 0.03%, more preferably 0.05%, or even more preferably 0.20%. It is preferable that the upper limit of the Si content be 0.7%, or more preferably 0.5%.

Mn: 0.1% to 2.0%

Mn is added for the purpose of increasing strength and toughness, and it is not possible to sufficiently realize such an effect in the case where the Mn content is less than 0.1%. On the other hand, in the case where the Mn content is more than 2.0%, there is a deterioration in weldability. Therefore, the Mn content is set to be 0.1% to 2.0%. Here, it is preferable that the lower limit of the Mn content be 0.3%, or more preferably 0.5%. It is preferable that the upper limit of the Mn content be 1.6%, more preferably 1.3%, or even more preferably 1.0%.

P: 0.003% to 0.03%

Since P deteriorates toughness and weldability, the P content is limited to 0.03% or less. Since there is a disadvantage from the viewpoint of dephosphorization cost in the case where the P content is excessively decreased, the lower limit of the P content is set to be 0.003%. Here, it is preferable that the P content be in the range of 0.003% to 0.025%, or more preferably 0.003% to 0.015%.

S: 0.01% or Less

S is an important chemical element which has an influence on the corrosion resistance of the steel according to aspects of the present invention. S is inevitably contained. In the case where the S content is large, there is a deterioration in toughness and weldability, and there is a decrease in corrosion fatigue resistance due to an increase in the number of inclusions such as MnS, which become the starting points at which corrosion fatigue occurs. Also, since inclusions, which become the starting points at which corrosion fatigue occurs, become preferential anode sites, pitting corrosion is promoted. Therefore, it is preferable that the S content be as small as possible, and it is acceptable that the S content be 0.01% or less, preferably 0.005% or less, or more preferably 0.003% or less. On the other hand, for the reasons described above, there is no particular limitation on the lower limit of the S content.

Al: 0.005% to 0.100%

Al is added as a deoxidzing agent, and there is a decrease in toughness due to an insufficient deoxidizing effect in the case where the Al content is less than 0.005%. On the other hand, in the case where the Al content is more than 0.100%, there is a decrease in the toughness of a weld metal in the case where welding is performed. Therefore, the Al content is limited to 0.100% or less.

In addition, Al has a function of further increasing the effect of increasing acid resistance through the use of Sb and Sn described below. That is, since Al³⁺ ions which have been eluted due to the anodic dissolution of a base metal undergo a hydrolysis reaction with water which exists in a small amount in bioethanol, the formation of Sb oxides and Sn oxides described below is promoted due to a decrease in pH at the anode site. Such an effect becomes noticeable in the case where the Al content is 0.005% or more. On the other hand, in the case where the Al content is more than 0.100%, since a decrease in pH at the anode site is strongly promoted, there is an excessive decrease in pH, which makes it difficult to sufficiently realize the effect of increasing corrosion resistance through the promoted formation of Sb oxides and Sn oxides. From the viewpoint of achieving satisfactory toughness and corrosion fatigue resistance at the same time, it is preferable that the lower limit of the Al content be 0.010%, more preferably 0.015%, or even more preferably 0.020%. Similarly, it is preferable that the upper limit of the Al content be 0.070%, more preferably 0.060%, or even more preferably 0.050% or less.

N: 0.0010% to 0.010% and 2.0≤Al/N≤70.0

N is an important chemical element which has an influence on the corrosion fatigue resistance of the steel according to aspects of the present invention. By decreasing the N content, since the formation of coarse nitrides is inhibited, there is an increase in corrosion fatigue life. On the other hand, in the case where the N content is more than 0.010%, since the formation of coarse AlN is promoted, it is not possible to sufficiently realize the above-described effect of increasing corrosion fatigue resistance through the use of Al, and there is an increase in corrosion fatigue sensitivity due to coarse AlN functioning as a starting point at which corrosion fatigue occurs. Therefore, the N content is limited to be 0.010% or less, preferably 0.007% or less, or more preferably 0.005% or less. In addition, N has an important function of stably realizing the above-described effect of increasing corrosion fatigue resistance through the use of Al. That is, while a decrease in pH due to the hydrolysis of Al³⁺ ions contributes to an increase in corrosion fatigue resistance through the promoted formation of Sb oxides and Sn oxides, there is a risk of a decrease in corrosion fatigue resistance eventually in the case where there is an excessive decrease in pH. Here, N in steel has a buffering function of inhibiting an excessive decrease in pH by consuming H⁺ to form NH₄ ⁺ through anodic dissolution. In order to realize such a buffering function, it is necessary that the N content be at least 0.0010% or more. Therefore, the lower limit of the N content is set to be 0.0010%, or preferably 0.0015%.

In addition, since Al and N are strongly related t.o each other, for example, in the formation of AlN and the realization of the effect of increasing corrosion fatigue resistance through the use of Al as described above, it is also important to appropriately control (the Al content)/(the N content) (mass ratio) in a steel material. In the case where the Al content is excessively large with relation to the N content, that is, in the case, where (the Al content)/(the N content) is more than 70.0, since there is a significant increase in the formation rate of AlN, there is a coarsening of AlN, and the buffering function through the formation of NH₄ ⁺ does not catch up with the formation of AlN. Therefore, the upper limit of (the Al content)/(the N content) is set to be 70.0, preferably 50.0, or more preferably 20.0. On the other hand, in the case where (the Al content)/(the N content) is less than 2.0, since most of the Al in steel exists in the form of AlN, there is an insufficient number at Al³⁺ ions formed due to the anodic dissolution of a base metal. That is, it is not possible to sufficiently allow Al to have a function of increasing the effect of increasing acid resistance through the use of Sb and Sn. Therefore, the lower limit of (the Al content)/(the N content) is set to be 2.0, preferably 3.0, or more preferably 5.0.

At least one selected from W: 0.010% to 0.5% and Mo: 0.010% to 0.5%

W is a chemical element which is effective for increasing corrosion fatigue resistance. Since W, like Mo, forms corrosion products such as oxoacid ions, and such corrosion products are rapidly adsorbed onto a crack tip to decrease anode reaction activity in the case where a crack, which becomes a starting point at which stress corrosion cracking occurs, is formed, W has a function of inhibiting the growth of the crack. In addition, as a result of W being taken in an oxide film on the surface of a steel material, since there is an increase in the dissolution resistance of the oxide film in an acidic environment of carboxylic acid, which is contained as an impurity in bioethanol, W is also effective both for decreasing the degree of inhomogeneous corrosion and for decreasing pitting corrosion resistance. However, in the case where the W content is less than 0.010%, it is not possible to sufficiently realize the effects of increasing corrosion fatigue resistance and pitting corrosion resistance. On the other hand, in the case where the W content is more than 0.5%, there is a disadvantage from the viewpoint of cost. Therefore, the W content is set to be 0.010% to 0.5%. It is preferable that the lower limit of the W content be 0.05%, or more preferably 0.08%. In order to prevent an increase in cost, it is preferable that the upper limit of the W content be 0.3%, or more preferably 0.2%.

Mo is a chemical element which is effective for increasing corrosion fatigue resistance. Since Mo forms corrosion products such as oxoacid ions, and such corrosion products are rapidly adsorbed onto a crack tip to decrease anode reaction activity in the case where a crack, which becomes a starting point at which corrosion fatigue occurs, is formed, Mo has a function of inhibiting he growth of the crack. In addition, as a result of Mo being taken an oxide film on the surface of a steel material, since, there is an increase in the dissolution resistance of the oxide film in an acidic environment of carboxylic acid, which is contained as an impurity in bioethanol, Mo is also effective both for decreasing the degree of inhomogeneous corrosion and for decreasing pitting corrosion resistance. However, in the case where the Mo content is less than 0.010%, it is not possible to sufficiently realize the effects of increasing corrosion fatigue resistance and pitting corrosion resistance. On the other hand, in the case where the Mo content is more than 0.5%, there is a disadvantage from the viewpoint of cost. Therefore, the Mo content is set to be 0.010% to 0.5%. It is preferable that the lower limit of the Mo content be 0.05%, or more preferably 0.08%. Moreover, in order to prevent an increase in cost, it is preferable that the upper limit of the Mo content be 0.4%, or more preferably 0.3%.

Here, in accordance with aspects of the present invention, it is preferable that W and Mo described above be added in order to achieve good corrosion fatigue resistance.

At least one selected from Sb: 0.01% to 0.5% and Sn: 0.01% to 0.3%

Sb is a chemical element which increases acid resistance and is an important chemical element which increases the corrosion fatigue resistance of the steel according to aspects of the present invention. In particular, Sb is a chemical element which is effective for inhibiting the growth of a crack at a corrosion fatigue crack tip, which is in a low-pH environment. Sb is retained and concentrated at an anode site in the form of oxides due to the anodic dissolution of a base metal. As a result, since the anode portion is protected, the progress of a dissolution reaction is strongly inhibited, which results in an increase in corrosion fatigue resistance. However, in the case where the Sb content is less than 0.01%, such an effect is insufficiently realized. On the other hand, in the case where the Sb content is more than 0.5%, a limitation is imposed from the viewpoint of manufacturing a steel material. Therefore, the Sb content is set to be in the range of 0.01% to 0.5%. Here, it is preferable that the lower limit of the Sb content be 0.02%, or more preferably 0.05%. It is preferable that the upper limit of the Sb content be 0.4%, or more preferably 0.30%.

Sn is, like Sb, a chemical element which increases acid resistance and is an important chemical element which increases the corrosion fatigue resistance of the steel material according to aspects of the present invention. In particular, Sn is a chemical element which is effective for inhibiting the growth of a crack at a corrosion fatigue crack tip, which is in a low-pH environment. Sn is retained and concentrated at an anode site in the form of oxides through the anodic dissolution of a base metal. As a result, since the anode portion is protected, the progress of a dissolution reaction is strongly inhibited, which results in an increase in corrosion fatigue resistance. However, in the case where the Sn content is less than 0.01%, such an effect is insufficiently realized. On the other hand, in the case where the Sn content is more than 0.3%, a limitation is imposed from the viewpoint of the manufacture of a steel material. Therefore, the Sn content is set to be in the range of 0.01% to 0.3%. Here, it is preferable that the lower limit of the Sn content be 0.02%, or more preferably 0.05%. It is preferable that the upper limit of the Sn content be 0.30%, or more preferably 0.15%.

Here, in accordance with aspects of the present invention, it is preferable that Sb and Sn described above be added from the viewpoint of achieving good corrosion fatigue resistance.

Among the constituent chemical elements described above, it is important to combine a highly rapid-acting surface-protection function through the use of Mo oxoacid ions and W oxoacid ions and a strong surface-protection function through the use of Sb oxides and Sn oxides in accordance with aspects of the present invention. That is, in the case where the growth rate of a corrosion fatigue crack is high, since the formation of Sb oxides and Sn oxides does not naturally catch up with the crack growth at the crack tip, it is not possible to realize the surface-protection function through the use of Sn and Sb in the crack portion. However, the case where Mo and W exist, the rapid surface-protection function through the use of Mo oxoacid ions and W oxoacid ions realized in the crack portion. With this, since there is a decrease in crack growth rate, the formation of Sb oxides and Sr oxides catch up with the crack growth at the crack tip. As a result, since the crack tip is covered with a strong surface-protection layer composed of two kinds of layers, that is, an oxoacid ion layer and an oxide layer, corrosion fatigue is strongly inhibited. Here, controlling the Al content and decreasing the N content are important for promoting the formation of Sb oxides and Sn oxides. Since decreasing the N content contributes to decreasing the number of starting points at which corrosion fatigue occurs, it is possible to realize a superimposed effect of increasing corrosion fatigue resistance.

The basic constituent chemical elements are described above, and the chemical elements below may further be added as needed in accordance with aspects of the present invention.

At least one selected from Cu: 0.05% to 1.0%, Cr: 0.01% to 1.0%, and Ni: 0.01% to 1.0%

Cu, Cr, and Ni are chemical elements which are effective for increasing corrosion fatigue resistance in an acidic environment of carboxylic acid, which is contained as an impurity in bioethanol. However, in the case where the contents of these chemical elements are small, there is no such effect. On the other hand, in the case where the content of each of these chemical elements is more than 1.0%, a limitation is imposed from the viewpoint of the manufacture of a steel material. Therefore, the Cu content is set to be 0.05% to 1.0%, the Cr content is set to be 0.01% to 1.0%, and the Ni content is set to be 0.01% to 1.0%. It is preferable that the upper limit of the Cu content be 0.5%, or more preferably 0.2%. It is preferable that the upper limit of the Cr content be 0.5%, or more preferably 0.2%. It is preferable that the upper limit of the Ni content be 0.5%, or more preferably 0.2%.

At least one selected from Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, and REM: 0.001% to 0.2%

As described above, since MnS becomes a starting point at which pitting corrosion and corrosion fatigue occur, MnS has a negative effect. Ca, Mg, and REM are chemical elements which are effective for decreasing such a negative effect through the control of the shape and dispersion of sulfides in steel. It is not possible to sufficiently realize such an effect in the case where the contents of these chemical elements are small. On the other hand, in the case where the contents of these chemical elements are large, Ca, Mg, and REM become coarse inclusions, which become the starting points at which pitting corrosion and corrosion fatigue occur. Therefore, the Ca content is set to be 0.0001% to 0.02%, the Mg content is set to be 0.0001% to 0.02%, and the REM content is set to be 0.001% to 0.2%. It is preferable that the lower limit of the Ca content be 0.001%. It is preferable that the upper limit of the Ca content be 0.005%. It is preferable that the lower limit of the Mg content be 0.001%. It is preferable that the upper limit of the Mg content be 0.005%. It is preferable that the upper limit of the REM content be 0.030%.

At least one selected from Ti: 0.005% to 0.1%, Zr: 0.005% to 0.1%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%

One, two or more selected from Ti, Zr, Nb, and V may also be added in order to improve the mechanical properties of steel. In the case where the content of each of these chemical elements is less than 0.005%, it is not possible to sufficiently realize such an effect. On the other hand, in the case where the content of each of these chemical elements is more than 0.1%, there is a deterioration in the mechanical properties of a weld zone. Therefore, the content of each of these chemical elements is set to be in the range of 0.005% to 0.1%, or preferably 0.005% to 0.05%.

The constituent chemical elements of the steel material according to aspects of the present invention other than those described above are Fe and inevitable impurities. Moreover, a constituent chemical element which is inevitably contained in addition to those described above may also be contained as long as it is in the range that there is no decrease in the effects of aspects of the present invention.

A pitting corrosion portion and a crack tip are particularly exposed to a low-pH environment in an ethanol solution containing 0.02 mmol/L or more of carboxylic acid, 0.02 mg/L or more of chloride ions, and 0.05 vol % or more of water. Therefore, embrittlement cracking due to secondarily generated hydrogen may occur in addition to the occurrence of pitting corrosion and cracking. In order to decrease the hydrogen embrittlement sensitivity of steel, it is preferable that the tensile strength and yield strength of the steel according to aspects of the present invention be respectively 825 MPa or less and 705 MPa or less.

The steel according to aspects of the present invention can suitably be used for storage equipment and transportation equipment for ethanol. In addition, the steel according to aspects of the present invention is steel excellent in terms of ethanol corrosion resistance which can be used in a corrosive environment in ethanol containing carboxylic acid, chloride ions, and water, in particular, in bioethanol.

The term “carboxylic acid” in accordance with aspects of the present invention means aliphatic carboxylic acid having a carbon number of 1 to 5. The term “storage equipment and transportation equipment for ethanol” in accordance with aspects of the present invention means equipment for, for example, storing, transporting, conveying, accumulating, distributing, recovering, or blending ethanol. Examples of such equipment include a tank, a steel pipe, a tanker, pipework, a pipeline, a nozzle, and a valve. Although the shape of the steel for storage equipment and transportation equipment for ethanol according to aspects of the resent invention may be selected as needed, it is preferable that the steel according to aspects of the present invention have a plate shape. It is preferable that the steel according to aspects of the present invention have a thickness (wall thickness) of 1 mm to 50 mm, more preferably 3 mm to 50 mm, or even more preferably 5 mm to 50 mm.

Hereafter, a preferable method for manufacturing the steel material according to aspects of the present invention will be described.

After having prepared molten steel having the chemical composition described above by using a known furnace such as a converter or an electric furnace, steel such as a slab or a billet is manufactured by using a known method such as a continuous casting method or an ingot-making method. Here, when molten steel is prepared, for example, vacuum-degassing refining may be performed.

The chemical composition of molten steel may be controlled by using a known steel-refining method.

Subsequently, when the steel described above is hot-rolled into desired size and shape, it is preferable that the steel be heated to a temperature of 1000° C. to 1350° C. In the case where the heating temperature is lower than 1000° C., since there is an increase in resistance to deformation, hot rolling tends to be difficult. On the other hand, in the case where the heating temperature is higher than 1350° C., there is a risk of surface defects, scale loss, or an increase in specific fuel consumption. It is preferable that the heating temperature be in the range of 1050° C. to 1300° C. Here, in the case where the temperature of the steel is originally in the range of 1000° C. to 1350° C., hot rolling may be directly performed without heating the steel.

Here, in hot rolling, the finishing delivery temperature of hot rolling is usually optimized. It is preferable that the finishing delivery temperature of hot rolling be 600° C. or higher and 850° C. or lower. In the case where the finishing delivery temperature of hot rolling is lower than 600° C., since there is an increase in rolling load due to an increase in resistance to deformation, there is a risk of difficulty in rolling operation. On the other hand, in the case where the finishing delivery temperature of hot rolling is higher than 850° C., it may be impossible to achieve the desired strength. It is preferable that cooling after finish rolling of hot rolling be performed by using a natural cooling method or an accelerated cooling method at a cooling rate of 150° C./s or less. In the case where accelerated cooling is performed, it is preferable that a cooling stop temperature be 300° C. to 750° C. Here, a reheating treatment may be performed after the cooling.

EXAMPLES

Hereafter, the examples of the present invention will be described. Here, the present invention is not limited to these examples. Here, in the description of examples, the combination of Table 1-1 and Table 1-2 are referred to as Table 1. The combination of Table 2-1 and Table 2-2 is referred to as Table 2.

After having prepared molten steel having the chemical compositions given in Table 1 by using a vacuum melting furnace or a converter, slabs were manufactured by using a continuous casting method. Subsequently, after having heated the slabs to a temperature of 1230° C., steel plates having a thickness of 15 mm was manufactured by performing hot rolling under the condition of a finishing delivery temperature of 850° C.

After having taken a micro tensile test piece (having a parallel part of 6 mmϕ×25 mm) in C-direction (the width direction) of the obtained steel plates as described above, a tensile test was performed at room temperature in accordance with JIS Z 2241 to derive yield strength (YS) and tensile strength (TS). The results are given in Table 1.

Moreover, a corrosion fatigue test was performed as described below.

First, a uniaxial round-bar-shaped tensile test piece (with a parallel part having a length of 25.4 mm and a diameter of 3.81 mmϕ) was taken from the steel plate, and the parallel part was then polished to #2000 finish. Subsequently, the test piece was subjected to ultrasonic degreasing in acetone for 5 minutes, subjected to air drying, and then fitted to a low-strain-rate tensile testing machine. A solution which had been prepared by adding 10 ml of water, 5 ml of methanol, 56 mg of acetic acid, and 13.2 mg of NaCl to 985 ml of ethanol was used as a simulated solution of bioethanol. The simulated solution of bioethanol was charged into a cell which covered the uniaxial round-bar-shaped tensile test piece, and a variable stress whose maximum value was 110% of yield strength (YS), which had been determined before the fatigue test was performed, and whose minimum value was 10% of the yield strength was appled the tensile axis direction of the uniaxial round-bar-shaped tensile test piece at a frequency, of 8.3×10⁻⁴ Hz for a maximum of 240 hours.

In evaluation, whether or not fracturing occurred during the testing period of time was first checked. Subsequently, in the case of a uniaxial round-bar-shaped tensile test piece in which fracturing had not occurred, the test piece was taken out of the cell after the test, and external observation was performed by using a microscope in order to check whether or not a crack was observed. In the case of a test piece in which a crack had been observed, the cross section in the tensile axis direction was observed in order to determine the maximum crack length in the cross section. Corrosion fatigue resistance was evaluated on the basis of the judgment criteria below. A case of a crack length of less than 20 μm was judged as a (satisfactory) case of slow crack growth and a low risk of corrosion fatigue fracturing occurring in practical equipment.

⊙: without a crack

◯: with small cracks (having a crack length of less than 20 μm)

Δ: with cracks (having a crack length of 20 μm or more)

×: fracturing

The obtained results are given in Table 2.

As Table 2 indicates, it is clarified that, in the case of all the examples of the present invention, there was a distinct improvement in the degree of corrosion fatigue cracking in the simulated solution of bioethanol. In contrast, in the case of all comparative examples whose chemical compositions were out of the range according to aspects of the present invention, fracturing occurred or the degree of corrosion fatigue cracking was high.

On the basis of a comparison between the examples of the present invention and the comparative examples, the present invention produces distinct improvement. In addition, from the results of Auger spectroscopic analysis performed on the crack tip of the example of the present invention in which a crack occurred, it was clarified that a surface layer which was composed of distinct two layers, that is, a layer in which oxoacid-ion-forming chemical elements (W and Mo) were concentrated and a layer in which oxide-forming chemical elements (Sn and Sb) were concentrated was formed at the crack tip. That is, in the case of the examples of the present invention, the crack tip was protected by a strong protection layer.

TABLE 1 Example Chemical Composition (mass %) No. C Mn Si P S Al N Mo W Sb Sn  1 0.05 0.91 0.32 0.011 0.0022 0.030 0.0035 0.1 — 0.08 —  2 0.05 0.95 0.28 0.012 0.0025 0.025 0.0033 0.05 — — 0.1  3 0.06 0.90 0.31 0.013 0.0019 0.030 0.0031 — 0.1 0.06 —  4 0.04 1.00 0.31 0.009 0.0022 0.030 0.0030 — 0.1 — 0.06  5 0.04 0.92 0.29 0.011 0.0019 0.029 0.0034 0.1 0.2 0.08 —  6 0.05 0.95 0.30 0.009 0.0020 0.030 0.0033 0.1 0.2 0.06 0.06  7 0.09 1.67 0.28 0.012 0.0027 0.027 0.0031 0.1 0.2 0.06 0.06  8 0.05 0.92 0.28 0.010 0.0020 0.027 0.0028 0.1 — 0.1 0.05  9 0.05 0.93 0.33 0.011 0.0021 0.027 0.0031 — 0.1 0.1 0.05 10 0.04 0.89 0.29 0.009 0.0023 0.028 0.0032 0.08 0.08 — 0.15 11 0.05 0.90 0.30 0.013 0.0020 0.030 0.0025 0.4 — 0.4 — 12 0.09 0.82 0.31 0.010 0.0021 0.028 0.0034 0.1 — 0.08 — 13 0.05 0.75 0.29 0.009 0.0021 0.029 0.0030 0.1 — — 0.1 14 0.04 0.80 0.28 0.012 0.0018 0.025 0.0029 — 0.1 0.06 — 15 0.05 0.77 0.30 0.011 0.0021 0.030 0.0028 — 0.1 — 0.06 16 0.06 0.94 0.31 0.009 0.0024 0.027 0.0028 0.2 — 0.1 — 17 0.06 0.91 0.33 0.013 0.0021 0.030 0.0033 0.25 — — 0.1 18 0.06 1.20 0.33 0.009 0.0021 0.032 0.0031 0.2 — 0.2 — 19 0.05 0.89 0.31 0.011 0.0019 0.030 0.0031 — 0.2 0.1 — 20 0.07 0.83 0.30 0.010 0.0023 0.055 0.0029 — 0.1 — 0.1 21 0.05 0.92 0.29 0.013 0.0021 0.030 0.0017 — 0.1 0.1 — 22 0.07 0.91 0.32 0.009 0.0020 0.029 0.0032 — 0.25 0.08 — 23 0.09 1.25 0.39 0.010 0.0020 0.030 0.0034 0.04 0.04 0.03 0.03 24 0.06 0.82 0.28 0.009 0.0023 0.091 0.0062 0.05 — 0.04 — 25 0.05 0.85 0.31 0.011 0.0021 0.018 0.0011 — 0.06 0.1 — 26 0.07 1.18 0.15 0.013 0.0035 0.011 0.0043 0.2 0.1 0.09 — 27 0.09 0.99 0.45 0.006 0.0008 0.088 0.0014 0.12 0.05 — 0.15 28 0.04 1.05 0.40 0.011 0.0041 0.079 0.0021 0.02 0.02 0.02 0.01 29 0.07 0.91 0.30 0.011 0.0021 0.028 0.0033 0.05 — — — 30 0.08 1.19 0.31 0.009 0.0023 0.033 0.0033 — 0.05 — — 31 0.09 0.89 0.29 0.008 0.0020 0.032 0.0033 — — 0.05 — 32 0.07 0.93 0.29 0.010 0.0022 0.025 0.0032 — — — 0.05 33 0.08 0.80 0.28 0.009 0.0019 0.030 0.0032 0.005 0.005 0.05 — 34 0.08 0.75 0.30 0.011 0.0020 0.027 0.0032 — 0.05 0.004 0.004 35 0.06 0.92 0.32 0.010 0.0020 0.038 0.011 — 0.05 — 0.05 36 0.05 0.86 0.31 0.009 0.0021 0.003 0.0028 0.05 — 0.05 — 37 0.09 0.91 0.30 0.010 0.0018 0.034 0.0028 — — — — 38 0.09 1.75 0.43 0.012 0.0029 0.025 0.0033 — 0.05 0.004 0.004 39 0.08 1.21 0.39 0.009 0.0021 0.089 0.0012 — 0.1 0.05 — 40 0.09 0.83 0.28 0.012 0.0023 0.011 0.0061 0.05 — — 0.08 41 0.05 1.55 0.22 0.015 0.0045 0.120 0.0035 — 0.05 — 0.15 42 0.05 1.13 0.44 0.010 0.0033 0.031 0.0008 0.15 — 0.05 — 43 0.09 1.06 0.35 0.008 0.0024 0.061 0.0020 — — — — Tensile Property Example Chemical Composition (mass %) (MPa) No. Cr Cu Ni Ca Mg REM Nb Zr V Ti Al/N YS TS Note  1 — — — — — — — — — — 8.6 415 539 Example 1  2 — — — — — — — — — — 7.6 430 550 Example 2  3 — — — — — — — — — — 9.7 441 551 Example 3  4 — — — — — — — — — — 10.0  430 567 Example 4  5 — — — — — — — — — — 8.5 467 570 Example 5  6 — — — — — — — — — — 9.1 483 585 Example 6  7 — — — — — — 0.05 — — 0.03 8.7 707 830 Example 7  8 — — — — — — — — — — 9.6 461 572 Example 8  9 — — — — — — — — — — 8.7 452 559 Example 9 10 — — — — — — — — — — 8.8 460 568 Example 10 11 — — — — — — — — — — 12.0  553 673 Example 11 12 0.05 — — — — — — — — — 8.2 512 630 Example 12 13 — 0.05 — — — — — — — — 9.7 409 506 Example 13 14 — — 0.05 — — — — — — — 8.6 398 510 Example 14 15 0.03 0.08 0.03 — — — — — — — 10.7  429 529 Example 15 16 — — — 0.002 — — — — — — 9.6 470 592 Example 16 17 — — — — 0.002 — — — — — 9.1 504 619 Example 17 18 — — — — — 0.008 — — — — 10.3  547 656 Example 18 19 — — — — — — 0.02 — — — 9.7 444 568 Example 19 20 — — — — — — — 0.02 — — 19.0  426 541 Example 20 21 — — — — — — — — 0.02 — 17.6  436 545 Example 21 22 — — — — — — — — — 0.02 9.1 513 636 Example 22 23 — — — 0.002 — — 0.04 — — 0.01 8.8 602 722 Example 23 24 — — — — — — — — — 0.01 14.7  455 549 Example 24 25 — — — — — — — — 0.02 — 16.4  435 529 Example 25 26 — — — — — — — — — — 2.6 499 592 Example 26 27 — — — — — — — — — — 62.9  469 581 Example 27 28 — — — — — — — — — — 37.6  455 558 Example 28 29 — — 0.03 — — — — — — — 8.5 431 545 Comparative Example 1 30 — — — — — — — — 0.02 — 10.0  501 602 Comparative Example 2 31 — — — — — — — — — — 9.7 445 570 Comparative Example 3 32 0.05 — — — — — — — 0.01 — 7.8 428 539 Comparative Example 4 33 — — — 0.002 — — 0.01 — — 0.01 9.4 430 543 Comparative Example 5 34 — — — — — — — — — — 8.4 417 514 Comparative Example 6 35 — — — — — — — 0.02 — — 3.5 418 533 Comparative Example 7 36 — — — — — — — — — — 1.1 396 500 Comparative Example 8 37 — 0.08 — — — — — — — — 12.1  402 509 Comparative Example 9 38 — — — — — — — — 0.03 0.02 7.6 712 838 Comparative Example 10 39 — — — — — — — — — — 74.2  588 690 Comparative Example 11 40 — — — — — — — — — — 1.8 479 582 Comparative Example 12 41 — — — — — — — — — — 34.3  625 719 Comparative Example 13 42 — — — — — — — — — — 38.8  490 601 Comparative Example 14 43 0.12 0.05 0.03 — — — — — — — 30.5  481 590 Comparative Example 15

TABLE 2 Maximum Crack Length Example No. Evaluation (μm) Note No. 1 ◯ 10 Examples No. 2 ◯ 11 No. 3 ◯ 12 No. 4 ◯ 12 No. 5 ⊙ — No. 6 ⊙ — No. 7 ◯ 10 No. 8 ⊙ — No. 9 ⊙ — No. 10 ⊙ — No. 11 ⊙ — No. 12 ◯ 6 No. 13 ◯ 6 No. 14 ◯ 8 No. 15 ◯ 6 No. 16 ◯ 7 No. 17 ◯ 5 No. 18 ◯ 3 No. 19 ◯ 8 No. 20 ◯ 8 No. 21 ◯ 4 No. 22 ◯ 6 No. 23 ◯ 10 No. 24 ◯ 11 No. 25 ◯ 10 No. 26 ◯ 8 No. 27 ◯ 7 No. 28 ◯ 18

TABLE 2-2 Example Maximum Crack No. Evaluation Length (μm) Note No. 29 Δ 58 Comparative No. 30 Δ 68 Examples No. 31 Δ 80 No. 32 Δ 71 No. 33 Δ 34 No. 34 Δ 49 No. 35 Δ 33 No. 36 Δ 42 No. 37 x — No. 38 x — No. 39 Δ 31 No. 40 Δ 32 No. 41 Δ 39 No. 42 Δ 41 No. 43 x — 

1. Steel for storage equipment and transportation equipment for ethanol, the steel having a chemical composition containing, by mass %, C: 0.02% to 0.3%, Si: 0.01% to 1.0%, Mn: 0.1% to 2.0%, P: 0.003% to 0.03%, S: 0.01% or less, Al: 0.005% to 0.100%, N: 0.0010% to 0.010%, at least one selected from W: 0.010% to 0.5% and Mo: 0.010% to 0.5%, at least one selected from Sb: 0.01% to 0.5% and Sn: 0.01% to 0.3%, and the balance being Fe and inevitable impurities, in which the ratio of the Al content to the N content satisfies the relationship 2.0≤Al/N≤70.0.
 2. The steel for storage equipment and transportation equipment for ethanol according to claim 1, wherein the steel has the chemical composition further containing, by mass %, at least one selected from Cu: 0.05% to 1.0%, Cr: 0.01% to 1.0%, and Ni: 0.01% to 1.0%.
 3. The steel for storage equipment and transportation equipment for ethanol according to claim 1, wherein the steel has the chemical composition further containing, by mass %, at least one selected from Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, and REM: 0.001% to 0.2%.
 4. The steel for storage equipment and transportation equipment for ethanol according to claim 1, wherein the steel has the chemical composition further containing, by mass %, at least one selected from Ti: 0.005% to 0.1%, Zr: 0.005% to 0.1%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
 5. The steel for storage equipment and transportation equipment for ethanol according to claim 1, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.
 6. The steel for storage equipment and transportation equipment for ethanol according to claim 2, wherein the steel has the chemical composition further containing, by mass %, at least one selected from Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, and REM: 0.001% to 0.2%.
 7. The steel for storage equipment and transportation equipment for ethanol according to claim 2, wherein the steel has the chemical composition further containing, by mass %, at least one selected from Ti: 0.005% to 0.1%, Zr: 0.005% to 0.1%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
 8. The steel for storage equipment and transportation equipment for ethanol according to claim 3, wherein the steel has the chemical composition further containing, by mass %, at least one selected from Ti: 0.005% to 0.1%, Zr: 0.005% to 0.1%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
 9. The steel for storage equipment and transportation equipment for ethanol according to claim 6, wherein the steel has the chemical composition further containing, by mass %, at least one selected from Ti: 0.005% to 0.1%, Zr: 0.005% to 0.1%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
 10. The steel for storage equipment and transportation equipment for ethanol according to claim 2, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.
 11. The steel for storage equipment and transportation equipment for ethanol according to claim 3, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.
 12. The steel for storage equipment and transportation equipment for ethanol according to claim 4, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.
 13. The steel for storage equipment and transportation equipment for ethanol according to claim 6, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.
 14. The steel for storage equipment and transportation equipment for ethanol according to claim 7, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.
 15. The steel for storage equipment and transportation equipment for ethanol according to claim 8, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.
 16. The steel for storage equipment and transportation equipment for ethanol according to claim 9, wherein the steel further has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less. 